In a study of cathepsin D structure and function, the enzyme from porcine spleen was purified to near homogeneity. The enzyme, which represents the major isoenzyme forms of this tissue, was found to be very similar to pepsin in molecular weight (35,000), specificity of peptides cleavage sites and amino acid composition. Current study includes the large scale purification method using affinity chromatography and the study of control of proteolysis of cathepsin D in the lysosomes.